Random idea combining #physics and #bitcoin:

I was trying to think of a way to encode ecash so you can send it over the public internet but in a way that could not be swiped by computers, only humans.

One idea I had was to tie a physical process into the decoding the qrcode: Encode the ecash into some complex pattern where if you using some kind of overlapping filter it would generate a reconstructed image of the qrcode. 

A long time ago I was trying to think of some polarizing filter scheme to do something similar. Maybe you could sell a physical polarization filter with some kind of moire pattern for decoding the image on your screen.

I like the idea of using optical phenomena to make it harder for a computer to simulate and decode. It would be a form of optical steganography(Is this a field?).

Not practical at all and not sure if it’s doable, but it’s a fun thought experiment.